Method of producing ultra-clean coal



, 2,842,319 METHOD OF PRODUCING ULTRA-CLEAN COAL Wilhelm Reerink, Essen-Bredeney, WalterMiischenbo'rn, Essen, and Erich Niitzold, Essen-Haarzopf, Germany No Drawing. Application October 21, 1953 Serial No. 387,538

Claims priority, application, Germany November 5, 1952 17 Claims. (Cl. 241-14) The invention relates to a method of producing ultra: clean coal by wet cleaning. v

All of the known methods to produce ultra-clean coal by wet processes have the disadvantage that the yield of such ultra-clean coal, i. e., of coal having ash content considerably less than 0.5 is rather small resulting in a correspondingly low economy of these methods. This disadvantage is also present in the conventional flotation process carried out with repeated treatment of the concentrates which may be subjected to comminution between the successive steps.

In contrast to the known wet methods, ultra-clean coal concentrates with an ash content substantially'below 0.5% are produced by the process according to the invention, whereby these concentrates are obtained in a considerable yield.

The new process is based on the discovery that the ultra: clean constituents to be separated from the coal, namely the pure vitrain components, and possibly clarains with low spore content, have a substantially'lower mechanical strength than the coal constituents having a higher ash,

content, i. e. the mineral constituents and, the coal constituents rendered impure by impregnation.

According to the basic principle .of the invention, the new process is carried out as follows: 1 The coal to be treated, i. e. either a fine disintegrated coal or, preferably, a suitable bed coal (lump coal) after being properly comminuted is classifiedto a suitable fine material size, whereupon the fine material from this classi ficat ion is subjected to flotation- Thereafter the -concentrate of this preliminary flotation is classified or screened again by means of a-finer mesh screen, the oversize grains from this classification are further comminuted and are subjected either together with the undersize grains from this classification, or separately therefrom, to one or several further flotation steps. I I

In this process the coarser grain fraction from the screening step preceding the preliminary flotation, on the one hand, and the oversize grain fraction fromthesubsequent screening step, i. e.-the constituents which primarily contain the minerally impregnated stronger grain on the other hand, are separated by mere mechanicalmeans and always the finest material containing the purest constituents is subjected to flotation, which may be repeated several times, if need. i

According to the present invention, the new process includes steps of selectively comminuting the starting coal vitrain particles of low ash content, thanthe mass of the refuse. In this process, the constituents obtained from veins having high durain and spore contents or from mineralized fractions are separated in the first screening step or sink directly in the preliminary flotation if they reach it. I I

In the preliminary flotation, the operation is preferablycarried out in such a manner that independently of the grain size of the material treated a positive separation between the purest particles raised into the concentrate and the impure grains to be rejected isobtained by adjusting the flotation range to approximately .75/mm, if

need to 1 mm. The oversize grain from the preliminary comminution, the size of which is above the selected flow tation limit and which contains the main portion ofthe, ash, can either be rejected at the outset or may be subjected to another comminution byrecycling it, while continnously separating the grain above a grain size found to be suitable, such as 3 mm. to avoid undersirably high losses of pure coal in the oversize grain. The grain between 1 and 3 mm. is further comminuted'and the fraction material according to the diiferent mechanical strengths of its petrographicconstituents. Thus, withluinp coal as a starting material, the preliminary comminutioni's prefer ably not carried out in such a manner that 'a grain size,

portant feature of the present'invention, is carrieduoutjn below 1 mm. of said comminuted material is subjected to flotation.

The comminution of the intermediate grain may be rred out separately by means of selective grinding, the

limit between the oversize grain to 'be'rejected and the fine material tobe subjected to flotation being successively lowered in each of the successive grinding steps. The comminution should-be conductedin such a manner so as to obtain a highly uniform grain distribution having a low percentage of very fine grain and a highoutput of gritty coarser grain. 1

. The impurities from the material obtained in this comminution step are separated by means of a flotation step carried out-in a very satisfactory manner, because the sc-' 7 lective separation takes place under the action of the higher weight of the harder, coarses and less bouyant fractions. Less effective flotation media 'are'advantagously employed in this operation, because the then resulting foam concentrates can subsequently be easily purified. I.

.Theconcentrates from the preliminary flotation maybe subjected to one 'or more after-flotations if. necessary. These concentrates represent a relatively coarse'fgrained'.

product havingan ash content in the range between' 0.8 to 1% which, in addition to ultra-cleanparticles of finest grian, generally contains extremely fine, highly dispersed mineral and very fin'efusain splinters having a high ash content, and in addition to these ordinaryash carriers, coarser particles having a high vitrain 'content coalesced with ash-containing particles or minerally impregnated particles.

The comminution and subsequent screening of this concentrate should be carried out to obtain agr'ai'n'siz'e of at least p, and if possible even'below that-size. "Of' course, this depends upon the nature of the 'coal'in' its raw state, though not to such extent that particles' of the refuse freed from'vitrain particles are unnecessarily split up, which would be disadvantageous to'the further The intern'rediate comminution may be effected in a bathe-plate mill or in grinding mills or the like. is followed by a screening step to separate the oversize grain, for example above 100 said grain having also a higher ash content. This screening step, according to an ima novel manner under addition. of largeamountsof'water by means of screen centrifuges with finely calibrated slots; 1

slits or holes- This is.particularlyadvantageous incase the extremely fine material sep'aratedlthereby is :-im-

' mediately subjected toga further-flotation, which has'to:

fiatented July 5,1958

'3. be carried out with very little solid pulp content due to the fineness of the material particles.

The oversize grain remaining in the centrifuge can either be recycled to the mill or returned to the preliminary flotation, or it may be entirely discarded. Pref-' erably, the oversize grain is subjectedto a further flotatiOn because it mostly contains a fractionof coarse vitrain particles freed by the comminution. This justifies the further flotation from an economical point of view.

According to another modification of the process according tothis invention, the centrifugal treatment may precede the comminution, i. e. it may take place immediately after the first flotation, whereupon only the coarse concentrate of the centrifuge is fed to the mill.

As compared with the first embodiment of the invention,

thecomminuted material is fed either to the same or to a separate flotation apparatus, treating only the latter material, to separate the freed impurities. The material may be returned to the screening centrifuge if desired.

Furthermore, it may be advantageous to treat the material which has been repeatedly ground in a separate centrifuge to avoid unfavorable repeated very fine grindingof the impurities. The coarse grain of this centrifuge is either discarded or returned to the preliminary flotation. V

Finally, the extremely finely ground refined material is subjected to a.final flotation, likewise with multiple repetition of'the concentrates, whereby the latter are yielded with an ash content in. the order of 0.3% and. lower, which heretofore could not be obtained by purely mechanical means.

During the individual flotation steps, the pH values which. were ascertained as the most favorable are preferably adjusted.

The wastes of the. last flotation constitute a concentrate having a very low ash content of. usually only 1 to 3% of ash, which. has commercial usefulness.

All of the coal-containing wastes of the process are preferably separated by. treating them with oil-containing substances for the purpose of phase reversal and simultaneous dispersion in water, followed by separation of the dispersing agent containing, thefine wastes from the coal. in a centrifuge so as to obtain pure wastes and ordinary commercial coal. 7.

According to a further development of the new process, products can be obtained which have properties particularlysuitable for special purposes. In this case, components which are adapted to acquire such qualities by being mixed with one another, for example concentrates composed of baking and nonbaking coals, are separately prepared and then mixed, whereby the components are preferably mixed prior to the dewatering step, and the mixture containing these components is filtered or advantageously centrifuged, resulting in an extremely intimate mixture.

It is also possible to first prepare a suitable mixture from the respective types of coal and then to subject the same to the treatment according to the new process.

If in such cases there is any risk that the mixture might disintegrate, it is recommended to first subject the components separately to preliminary flotation and to bring them together in the course of the intermediate comminution, so that in the subsequent flotation the components are treated together in the mixture. step, there is no danger that the mixture may disintegrate.

During' this 4' Such operations do not result in an enlargement of the flotation apparatus, since the amount of material passing through the flotation in any case requires a certain number of cells.

In order to impart to the prepared concentrates certain properties desired for their special purpose or use, they will be subjected, for example, to a coking treatment or low temperature carbonization, and hydrocarbons, such as pitches, oils, tars, or the like may be added to these concentrates. Such operation is carried out in a suitable apparatus in which the material is exposed to pressures and/or vacuum resulting displacement of the water layers from the surfaces of the coal particles by the hydrocarbons, whereby the latter become intimately bonded to the coal particles. The particles thus hydrophobised can be dewatered in a very satisfactory manner in screen-type centrifuges, whereby no coal particles will pass into the filtrate water, i'. c. any particles of refuse dispersed in the water will pass'through the screens. As a result of this, the ashes are removed to a still greater extent.

Ultra-clean coal mixed with pitches in this way can be coked in a superior manner, even if the coal itself is not cokable.

In this case it may be to further advantage to briquet the material prior to the coking step, such briquetting operation being readily carried out with coal concentrates treated with hydrocarbons, because these coal concentrates can be compressed into very strong briquettes without previous drying. These briquettes, when cokcd, result in extremely homogeneous bodies having excellent properties.

A numerical example of the results obtained with the new process is given below:

This invention is obviously subject to considerable modification and hence we do not choose to be limited to the examples disclosed herein but rather to the scope of the appended claims.

We claim:

l.- A method of producing ultra-clean coal having a very low ashcontent comprising in combination the steps of selectivelycomminuting a starting coal according to the: different mechanical strengths of its petrographic constituents, screening said comminuted material by means of a first screen, subjecting the fine material from said. screeningzstep: to a' preliminary flotation, screening the concentrate fronrsaid preliminary flotation step by meaus of a second screen of finer mesh than said first screen, further'breaking-up the oversize grain from said finer meshscreeningstep,..subjecting the undersize grain from said finer mesh screening step and said further broken-up, oversize grain from. said finer mesh screening step to a further flotation and collecting the concentrate from said further flotation step- 21 A method of] producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a starting coal according. to

the different meehanicalstreugths' ofits petrographic conconcentrate from said preliminary" flotation step by means of a second screen of finer mesh than said first screen, further breaking-up the oversize grain from said finer mesh screening step, subjecting the undersize grain from said finer mesh screening step and said further broken-up oversize grain from said finer mesh screening step jointly to a further flotation and collecting the concentrate from said further flotation step.

3. A method of producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a starting coal according to the different mechanical strengths of its petrographic constituents, screening said comminuted material by means of a first screen, subjecting the fine material from said screening step to a preliminary flotation, screening the concentrate from said preliminary flotation step by means of a second screen of finer mesh than said first screen, further breaking up the oversize grain from said finer mesh screening, subjecting the undersize grain of said finer mesh screening step and said further broken-up oversize grain from said finer mesh screening step separately to further flotations and collecting the concentrate from said further flotation steps.

4. A method of producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a starting coal according to the different mechanical strengths of its petrographic constituents, screening said comminuted material by means of a first screen, subjecting the fine material from said screening step to a preliminary flotation, subjecting the concentrate from said preliminary flotation step to a further breaking-up operation, screening said latter broken-up concentrate by means of a second screen of finer mesh than said first screen, further breaking-up the oversize grain from said finer mesh screening step, subjecting the undersize grain of said finer mesh screening step and said further broken-up oversize grain from said finer mesh screening step to a further flotation and collecting the concentrate from said further flotation step.

5. A method of producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a starting coal according to the different mechanical strengths of its petrographic constituents, screening said comminuted material by means of a first screen, subjecting the fine material from said screening step to a preliminary flotation, screening the concentrate from said preliminary flotation step by means of a second screen of finer mesh than said first screen, further breaking-up the oversize grain from said finer mesh screening step, subjecting the undersize grain from said finer mesh screening step to a further flotation,

further screening said further broken-up oversize grain from said finer mesh screening step, subjecting the undersize grain from said further screening step to a further flotation and collecting the concentrates from said further flotation steps.

6. A method of producing ultra-clean coal having a very low ash content according to claim 1 characterized by very finely comminuting the coarser grain from said first screening step of the comminuted starting coal and subjecting the comminuted material to the same treatment as the finer grain after the first screening step.

7. A method of producing an ultra-clean coal having a very low ash content according to claim 1 characterized by subjecting the oversize grain from said first screening step of the comminuted starting coal to the same flotation treatment as the undersize grain.

8. A method of producing ultra-clean coal having a very low ash content according to claim 5 characterized by subjecting the oversize grain from said further screening step to the same flotation treatment as the undersize grain.

9. A method of producing ultra-clean coal having a very low ash content according to claim 5 characterized by subjecting the oversize grain from said second screening step to independent screening and subjecting the oversize grain separated thereby to flotation treatment,

j 10. A method of producing ultra-clean coal having a very lovv ash content comprising in combination the steps of screening a fine coal produced by selective decomposition into components of different strengths, subjecting the fine material from said screening step to a preliminary flotation, screening the concentrate from said preliminary flotation step by means of a second screen of finer mesh than said first screen, further breaking-up the oversize grain from said finer mesh screening step, subjecting the undersize grain from said finer mesh screening step, and further broken-up oversize grain from said finer mesh screening step to a further flotation and collecting the concentrate from said further flotation step.

11. The method of producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a starting coal according to the different mechanical strengths of its petrographic constituents, screening said comminuted material by means of a first screen, subjecting the fine material from said screening step to a preliminary flotation, centrifugally sifting the concentrate from said preliminary flotation step, further comminuting the oversize grain from said centrifugal sifting step, subjecting the undersizegrain from said centrifugal sifting step and said further comminuted oversize grain from said centrifugal sifting step to a further flotation and collecting the concentrate from said further flotation step.

12. A method of producing ultra-clean coal having a very low ash content according to claim 1 characterized by oil-wetting said oversize grain with simultaneous dispersion in water, and mechanically separating the oilwetted mass.

13. A method of producing ultra-clean coal having a very low ash content comprising in combination the steps of selectively comminuting a mixture of diflerent types of coal selected in accordance with the required properties of the ultra-clean coal concentrates to be produced, screening said comminuted material by means of a first screen, subjecting the fine material from said screening step to a preliminary flotation, screening the concentrate from said preliminary flotation step by means of a second screen of finer mesh than said first screen, further breaking-up the oversize grain from said finer mesh screening step, subjecting the undersize grain from said finer mesh screening step and said further brokenup oversize grain to a further flotation and collecting the concentrate from said further flotation step.

14. A method of producing ultra-clean coal having a very low ash content according to claim 5 characterized by mixing said concentrates and subsequently dewatering them.

15. A method of producing ultra-clean coal having a very low ash content according to claim 5 characterized by mixing the concentrate from said preliminary flotation step with said further comminuted oversize grain and subjecting the mixture to after-flotation.

16. A method of producing ultra-clean coal having a very low ash content according to claim 1 characterized (References on following A page) References Cited in the file of this patent STATES PATENTS Bradford Oct. 21, 1873 Callow Mar. '12, 1918 5 Lehmann Apr. 19, 1932 Crawford Nov. 8, 1938 Kuhlwein May 22, 1945 8 OTHER REFERENCES United Stats Bureau of Mines Information Circulu' 7 481, 'Gtztober 1945, Methods of Producing "Ultra-clean Coal for Electrode Carbon in Germany, 13 pages. 

